Intrapartum sonography – eccentricity or necessity?

Abstract

Ultrasonography has been extensively used in obstetrics and gynecology since 1980's. It found application in pediatric gynecology, procreation period, post-menopause, pregnancy monitoring and after delivery. Although the first reports on the use of ultrasonography in assessing delivery mechanism were published in 1990's, yet to date labor progress is evaluated by means of physical examination in most delivery units. Intrapartum sonography is not routinely performed despite the fact that numerous studies documented high error rates of conventional obstetrical examination. Even an experienced physician makes a mistake in every third case of the fetal head descent and fontanelle position assessment. Nowadays, obstetrician's role is not to strain for vaginal delivery at all costs, but to provide the patient in labor and her newborn with maximal safety. To achieve this objective, an obstetrician should distinguish between women who will deliver spontaneously and whose who require Cesarean section. Proper decision should be made on the basis of objective and valid evaluation of obstetric setting, which cannot be achieved solely with physical examination. Intrapartum sonography was shown to be far more accurate than digital examination. Moreover, it is not technically demanding, provides high reproducibility and neither increases the rate of ascending infection or causes discomfort to the patient. Current research suggests that if used routinely, intrapartum sonography can increase the safety of labor and reduce cesarean section rate.

Introduction

Over the centuries obstetrics was a purely interventional medical discipline, either a craft or an art, aimed at helping the pregnant to deliver a baby. This vocation was firstly held by midwives, then by physicians, initially in private houses, then in hospitals. In the past, obstetricians were forced to perform heroic procedures to save the pregnant and her fetus. Unfortunately, not always successfully as shown by high mortality rates of both. The pregnancies were not supervised by a physician and the pregnant women were left with no attention until labor pains occurred. Not so long ago, the pregnancies have started to be controlled, the heart rate monitored and the fetal wellbeing evaluated.

Ultrasonography (US), and resultant ability to visualize directly the womb and the fetus, were implemented to obstetrical practice in 1980's⁽¹⁾. More and more commonly used, ultrasonography has become a basic tool for gynecologists and obstetricians. Due to the introduction of ultrasonography, progress in anesthesiology and surgical techniques, the strain of obstetrics was moved from the delivery suites to the pathology of pregnancy and sonographic department. Nowadays, ultrasonography plays a key role at every stage of woman life – it is used in pediatric gynecology, therapy of infertility and pregnancy screening, as well as post-partum and in post-menopausal patients. Ultrasonography is an excellent adjuvant for, often challenging, clinical gynecological examination. While 15 years ago, the size of the ovarian tumor was commonly compared to fruits, e.g. an orange or tangerine, or to the size of man's or woman's fist. Presently, in the 21^st century, it seems ridiculous due to the availability of a tool that enables one an accurate evaluation of the size and topography of every lesion within reproductive organs. Now we cannot imagine the qualification for surgery or even minor gynecological procedure without a prior ultrasonography. Using sonography during pregnancy, we can monitor fetal development and well-being. Moreover, we can diagnose ectopic pregnancies, abnormal fetal growth or fetal demise. At present, it is extremely unusual for the pregnant to be admitted with bleeding due to ruptured ectopic pregnancy, which was quite common in the past. Ultrasonography enables us to predict many pathologies, such as preeclampsia, fetal hypotrophy, chromosomal aberrations and pre-term labor. We can also detect cervical insufficiency, fetal macrosomia, placenta and vasa previa, and placenta increta. We are able to assess retained placenta and postpartum laceration of pelvic diaphragm. Ultrasonography is actually used everywhere except from the delivery suite. It is quite peculiar, isn't it?

In most delivery units around the world, patients are examined solely by palpation. Digital examination is, and will certainly remain for long, a gold standard, as obstetricians are invariably trained in digital internal and external examinations. It will take time to change. Digital examination is challenging and one should perform hundreds of exams for the patient's safety. Leopold's maneuvers, common way to determine the position of fetal head and pelvis, is quite straightforward, contrary to the assessment of fontanels and distance between the leading part and interspinal line. Furthermore, digital vaginal examination has many limitations. It is subjective and depends mainly on the examiner's experience and examination conditions (e.g., cervical dilation). Vaginal examination becomes inaccurate in the case of caput succedaneum and molding or asynclitism (non-axial presentation of fetal head). Several studies showed that digital vaginal examination is highly inaccurate, both in the evaluation of fetal occiput position and fetal head station. Importantly, erroneous results are obtained not only by residents by also by attending physicians.

In one of previous studies⁽²⁾, 496 women in the 1^st and 2^nd stage of labor were subjected to digital examination and ultrasonography. The result of digital examination regarding fetal occiput positon was inconclusive in 34% of the cases, and in 52% of the patients the results were highly inappropriate as fetal occiput position differed by more than 45° as compared to objective ultrasonographic assessment. In another study including a total of 214 women in labor^{(3, 4)}, the results of digital and ultrasonographic examination differed by at least 45° in nearly half (46%) of the cases. The accuracy of digital examination documented in another study of 148 pregnant women was even worse⁽⁵⁾. While discrepancies between the results of digital and ultrasonographic examination during the 1^st stage of labor corresponded to only 31% (!), they reached up to 65% during the 2^nd stage. In another study, 32 residents and 25 attending physicians were subjected to test with a birth simulator⁽⁶⁾. The participants were asked to assess fetal head in terms of its engagement (engaged in the pelvis or not), station (high, mid, low and outlet) and distance from the interspinal line (in cm). Depending on fetal head position, residents and attending physicians made “numerical” errors regarding the distance from interspinal line in 50–88% and 36–80% of the cases, respectively. Mean error rate for head station assessment (high, mid, low and outlet) was 30% (95%, CI: 25–35%) for residents and 34% (95%, CI: 27–41%) for attending physicians. The majority of errors pertained to wrong evaluation of fetal head at mid station instead of high station. The engagement of fetal head was misdiagnosed in ca. 12% of the cases. The error rate in evaluating fontanels was shown to correlate inversely with cervical dilation, amounting to 80% for small dilation and >60% for 8-10 cm dilation⁽²⁾. Other studies showed that the risk of failure during digital examination at full cervical dilatation approximates 25–30%^{(5, 7)}.

The use of intrapartum sonography

What are the practical benefits of intrapartum sonography? For many of us, the most obvious ones include estimation of fetal weight and fetal Doppler velocimetry. Although theoretically achievable, these tests are difficult to perform in clinical practice during delivery due to several reasons. The examination should be performed early, prior to or at the very beginning of labor. Even most basic measurements are challenging during regular uterine contractions, let alone fetal Doppler velocimetry when fetal head is engaged in the small pelvis. Fetal well-being is generally evaluated by means of cardiotocography, and Doppler ultrasound is not routinely used during labor except from clinical trials.

The term “intrapartum ultrasound” (ITU) refers exclusively to examination verifying correctness of the delivery mechanism. Initial reports on the application of intrapartum sonography were published in 1990's⁽⁸⁾, when it was used to assess fetal head station. During recent years, this method has started to be in the center of interest. Several studies showed that intrapartum sonography is superior to clinical examination in various aspects. Ultrasonographic evaluation of fetal head position and engagement is far more accurate than digital examination. Intrapartum sonography is quite straightforward, has flatter learning curve than digital examination and, importantly, provides objective and reproducible results ^(9–11). It is neither time consuming nor causes discomfort of patient⁽¹²⁾. The use of ultrasonography for pregnancy monitoring was reflected by a decrease in the number of digital examinations, and thus lower rate of ascending infections, especially in women with ruptured membranes. Furthermore, ultrasonography provides objective documentation of labor, which seems to be of vital importance in the present era of suing.

In contrast to common ultrasonography performed in pregnancy, intrapartum sonography does not require an advanced equipment or special training. It is a standard examination “at patient's bedside”. Using ultrasonography, an obstetrician can assess fetal position, situation, presentation and head station. Several pelvic measurements can be taken as well. Using intrapartum sonography, one can estimate the likelihood of natural delivery in primipara, objectively monitor the stage of labor, decide on the mode of delivery in the case of prolonged labor, and assess the conditions for operative vaginal delivery. Moreover, intrapartum ultrasound can be used for immediate verification of clinical diagnosis, just as in the case of ultrasonography performed in a doctor's office. Training in intrapartum sonography is much shorter than in the case of clinical examination. One study⁽¹¹⁾ compared learning curves for these two methods. A total of 100 digital vaginal examinations and 99 intrapartum sonographies were performed at >7 cm dilation. The group of examiners included midwifery students with no experience in digital examination and ultrasonography. Then their results were verified by experienced examiners. The error rate for the first 50 digital examinations performed by students approximated 50% and then decreased gradually, reaching plateau at 82 examinations. Overall error rate for digital examination was 30%. The most common inconsistencies were critical errors in fontanel assessment. The learning curve for ultrasonography reached its plateau earlier, at 32 examinations. Error rate was lower and the errors were less significant than in the case of digital examination. Overall error rate for sonography was 8%, and did not exceed 45° except from one case. Ultrasonography was shown to be highly reproducible, with intra-observer and inter-observer variability for the angle of progression smaller than 3°^{(4, 7)}.

Intrapartum sonography technique

A simple, portable ultrasonograph with a convex probe is suitable for intrapartum sonography. The examination is performed with the patient in a supine position and with emptied bladder.

During the first stage of examination, fetus orientation, presentation and position are assessed. A transabdominal ultrasound is used, as for a regular examination in pregnancy. If fetal head is still visible above the symphysis, localization of fontanels can be assessed on the basis of the occiput position (Fig. 1). Also an angle between the fetal spine and occiput can be determined; an extended head detection can be helpful in diagnosing face presentation.

Fig. 1.

Transabdominal scan. Assessment of fetal occiput topography. Posterior occipital position, fetal eyeballs can be seen above the symphysis

During the second stage, fetal head engagement is assessed. It is usually done when fetal head is no longer visible above the symphysis. A transperineal ultrasound is then used with a transducer placed on the perineum. A transducer should be protected with a sterile cover and covered with gel, and a probe should be placed interlabially, parallel to labia majora. Importantly, a transducer should be rotated clockwise from transverse to longitudinal position in such way that the right side of the probe is placed inferiorly, in the perineal region, with the left side of the probe, i.e. the one on which the thumb is kept, inferior to the clitoris. Subsequently, the transducer is moved gently to visualize long axis of the symphysis – the pubis should be visible on the left side (i.e. on top of the screen). In this setting, one can visualize fetal head below the symphysis, with vagina and uterine cervix localized in front of the fetal head (Fig. 2). Although some may consider this description unnecessary, yet serious mistakes in fetal head engagement can be made if the probe is rotated inappropriately. Using freeze frame, several measurements can be taken, including several clinically vital parameters: angle of progression (AoP), head direction (HD) and head–symphysis distance (HSD). Caput succedaneum, molding and asynclitism can be easily observed on a single image (cross-section) (Fig. 3)

Fig. 2.

Transperitoneal longitudinal scan. First stage of labor

Fig. 3.

Transperitoneal longitudinal scan. Second stage of labor

In order to measure the distance between fetal head and perineum, a transducer should be placed transversely on labia majora, in the vicinity of posterior frenulum. To obtain more reproducible measurements, the strain should be put on the probe until resistance of pelvic bones is perceived. Labia, vagina, uterine cervix and fetal head, but not symphysis, should be visible on the screen. Using freeze frame, one can determine head–perineum distance (HPD) and midline angle (MLA). Cervical dilatation (CD) can be assessed by placing the probe transversely to the labia, and subpubic arch angle (subpubic arch) can be measured if the probe is moved below the symphysis.

The angle of progression (Fig. 4) is formed by the line parallel to the long axis of the symphysis and the line tangential to the fetal head. It constitutes an objective, reproducible, noninvasive and easy to determine measure of fetal head engagement^{(13, 14)}. Angle of progression can be used for accurate evaluation of fetal head station, as well as to predict the mode of delivery and time of parturition. An analysis of computer-generated 3D images of the skeletal pelvis and the true images obtained during intrapartum sonography showed that interspinal line is situated 3 cm distally to the ultrasonographically-determined line being perpendicular to the inferior edge of symphysis (subpubic line). Measuring a distance between the leading point and subpubic line, one can determine the distance between the leading point and interspinal line. If the fetal head is engaged in high station (leading point on the level of interspinal line, station 0), the angle of progression approximates 116°⁽¹⁵⁾ (Fig. 5).

Fig. 4.

Transperitoneal longitudinal scan. Angle of progression determination

Fig. 5.

Transperitoneal longitudinal scan. Distance between subpubic line and interspinal line. Subpubic line (in blue) runs perpendicular to inferior edge of the symphysis (level -3). Red line: a line running at the level of the ischial spines (level 0)

Not surprisingly, the greater the angle of progression, the more likely spontaneous delivery. In primipara, fetal head should descent to pelvic cavity several weeks before labor, and the angle of progression at the time of labor should exceed 95°. The angle of progression >110° during the first stage and >120° during the second stage is a predictor of spontaneous delivery^(16–18). In one study,⁽¹²⁾ spontaneous delivery occurred in all patients in whom the angle of progression exceeded 120° during the second stage. It is not surprising, as such value means that fetal head has already descended to pelvic cavity. The angle of progression <138° at qualification for operative vaginal delivery can be used to predict (and avoid) a difficult forceps birth (sensitivity 85.7%, specificity 100%)⁽¹⁹⁾.

Head direction (Fig. 6) is defined as the direction of the perpendicular to the widest fetal head diameter line to the subpubic line. This parameter is postulated to be a quick and easy predictor of successful vaginal delivery in cases in which the second stage is prolonged. Uncomplicated vacuum delivery is likely if the “head up” sign occurs, and low if the “head down” sign is observed or fetal head is directed horizontally to patient's bed⁽¹⁶⁾.

Fig. 6.

Transperitoneal longitudinal scan. Orientation of fetal head (“head-up” sign)

Head–symphysis distance (Fig. 7) is the distance between the inferior edge of the symphysis and the closest point of fetal head on the perpendicular to the symphysis⁽²⁰⁾. The head-symphysis of ca. 2 cm corresponds to fetal head at high station. HSD >2 cm at early active phase of the second stage suggests that this stage will last longer than 60 min, 1.5 h on average⁽²¹⁾.

Fig. 7.

Transperitoneal longitudinal scan. Determination of head-symphysis distance

Head–perineum distance (Fig. 8) is the shortest distance between the perineal skin and fetal head (skull). Contrary to digital examination, which is frequently hindered due to presence of caput succedaneum, this parameter can be easily measured by means of ultrasonography (Fig. 2). In one study, spontaneous delivery occurred in more than 90% of primiparas with prolonged labor, head-perineum distance below 4 cm and the angle of progression >110°. Sonographic evaluation of head-perineum distance was also shown to be an accurate predictor of vaginal delivery after induction, with predictive value similar to that for ultrasonographic assessment of the cervix assessment and Bishop's score⁽²²⁾.

Fig. 8.

Transperitoneal transverse scan. Determination of head-perineum distance

Midline angle (Fig. 9) is formed by the midline of fetal head and the anteroposterior dimension of the pelvis. Midline angle <45° corresponds to internal rotation and ≥3 cm engagement of fetal head in relation to the interspinal line⁽²³⁾.

Fig. 9.

Transperitoneal transverse scan. Determination of midline angle

Sonography can be used to assess cervical dilation (Fig. 10). One study showed that the difference between cervical dilation evaluated by digital examination and sonography is smaller than 1 cm (0.8 cm)⁽²⁴⁾.

Fig. 10.

Transperitoneal transverse scan. Assessment of cervical dilation

Evaluation of subpubic arch (Fig. 11) is an important, yet extremely difficult to objectively assess, parameter. The assessment is quite simple if the transducer is placed transversely and inferiorly to the symphysis. The likelihood of spontaneous delivery was shown to decrease if the subpubic arch was smaller than 120–110°⁽²⁵⁾.

Fig. 11.

Transperitoneal transverse scan. Determination of subpubic angle

Occiput posterior delivery

A diagnosis of occiput posterior delivery is an important clinical issue. As widely known, it often results in delivery obstruction, low vaginal delivery rate (25% in primiparas and 50% in multiparas) and higher (2- to 4-fold) rate of instrumental deliveries. Moreover, it is markedly more often associated with severe perineal tear; the risk for grade 3 perineal tear during occiput posterior delivery is up to 3-fold higher than for anterior occipital position. Furthermore, occiput posterior delivery is associated with more significant loss of blood (usually >500 ml) and significantly higher neonatal complication rate (e.g. the risk of encephalopathy is 4 times higher)⁽²⁶⁾. Unfortunately, as mentioned above, posterior occipital position is often misdiagnosed during digital examination. In turn, it can be easily detected ultrasonographically: when the transducer is placed above the symphysis, fetal orbits and eyeballs can be seen instead of the occiput and cerebellum.

As every third fetus resumes posterior occipital position at the onset of labor, but most of them will change it thereafter, location of fetal occiput determined ultrasonographically at early stages should not be considered a predictor of delivery mode⁽²⁶⁾. Only 7–8% of fetuses remain in posterior occipital position during the second stage. How these cases can be identified? In order to estimate the likelihood of spontaneous posterior rotation of fetal occiput, also the position of fetal spine should be determined. The likelihood of the spontaneous rotation is 80-90% if the occiput is located posteriorly and the spine laterally or anteriorly. However, if both the occiput and spine are located posteriorly, the probability of rotation is markedly lower (ca. 20%). Monitoring posterior occipital delivery, one should consider differences in its mechanism. Head progression distance seems to be most accurate measure of posterior occipital delivery progress.

Head progression distance (Fig. 12) is the length of line being perpendicular to subpubic line, extending to distal part of fetal head⁽²⁷⁾. It generally represents a real track that fetal head takes in birth canal; however, until the hypomochlion leans on the inferior pubic symphysis edge and the occiput is expulsed, the angle of descend increases less noticeably that in anterior occipital position, while the head-symphysis distance raises.

Fig. 12.

Transperitoneal longitudinal scan. Head progression distance

The sonopartogram

In some delivery units, in which intrapartum sonography is a routine practice, sonopartograms, i.e. ultrasonographic parameters of labor progress (e.g. caput succedaneum, molding, cervical dilation and head-perineum distance) are recorded, rather than the results of common digital examination⁽²⁸⁾. Sonopartogram reflects patient's status before admission to the delivery room; aside from the abovementioned parameters, it also includes information on amniotic fluid volume (depth of the largest pouch) and umbilical artery flow.

Conclusion

The world is changing, and so is obstetrics. We should accept this fact and try to keep up, approach the new, implement new technologies. We can pretend that everything is how it “used to be”, perform digital examination with closed eyes and compare tumors to fruits, dilation to the number of fingers, thus wasting an opportunity that ultrasonography is trying to give our specialty. We should remember that the one who does not move forward, makes a step back. We can like it or not, a classic ars obstetrica is vanishing. Complicated vaginal procedures, such as manually assisted delivery, rotation or fetotomy, are no longer performed in obstructed labor. The principal aim is to deliver a baby spontaneously, safely above all. If the labor prolongs or fetus is under threat, obstetrician decides whether it still should be delivered vaginally, or rather by Cesarean. The status of a neonate delivered instrumentally (vacuum, forceps) to a large extent depends on correct assessment of obstetrical setting prior to the procedure. Patients who were inappropriately qualified for vaginal instrumental delivery (too high position of fetal head in the pelvis, erroneous evaluation of fontanels), are at increased risk of complications after multiple vacuum traction or failed forceps procedure.

Another example from everyday practice: at present, pelvic dimensions are determined with the use of pelvimeter in most delivery units. Many of us do take this measurement without strong belief that it matters in establishing the delivery mode in cases other than narrow pelvis. As widely known, clinical confirmation of fetal head engagement, being a kind of “internal pelvimeter”, is the only criterion confirming that bony birth canal is wide enough for a fetus⁽²⁹⁾. Perhaps we should assess the angle of progression in primipara? This measurement provides us with an information on fetal head station, reflecting the likelihood of spontaneous delivery, and indirectly also birth canal size.

Sonography really facilitates our practice, thus we hope that even experienced obstetricians, who do not use this method on a regular basis, will start to do so, as an adjunct to digital examination.

Numerous studies confirmed practical application of intrapartum sonography. Not only sonographic monitoring of labor seems to improve the safety and patient comfort, but also decreases the Cesarean section rate⁽³⁰⁾.

Undoubtedly, intrapartum sonography seems to be complicated, at least at current stage. Too many various measurements are proposed in literature and it is unclear which of them should be taken in everyday practice. Probably, one or two parameters of greatest clinical significance will be identified in near future. For the time being, we can, and should conduct our own studies aiming at developing intrapartum sonography standards.

Despite this illusive chaos, an attempt to “place a transducer” ourselves is worth making – only then it will occur that the image is really readable and the abovementioned measurements are intuitive and reflect what we examine digitally. When ignoring these measurements, an advantage of dynamic examination in ultrasonography can be applied, while placing a probe on the perineum enables us to follow fetal head station during uterine contractions in real-time.

Conflict of interest

None.